The field of the invention is that of electron projection lithography, or other nanotechnology fields, in particular holders for masks or other workpieces used in connection therewith.
Electron Projection Lithography (EPL) is one of the leading candidates for Next Generation Lithography, generally considered to be linewidths of 65 nm and below.
Other candidate technologies are electron pencil beams in a direct write mode and X-ray lithography. In the former the very thin electron beam is focused to the required size beamspot and is focused directly on the wafer. This technology is used currently and has been used for some time in writing masks (more formally termed reticles) for optical lithography. In the latter, the X-ray beam is spread out to the dimension of the chip being exposed and a method similar to contact printing is used. This type of printing is used because X-Rays cannot be focused. Since the mask and final image are in a 1:1 ratio, accuracy requirements in making the mask are extreme.
Electron Projection Lithography has the benefit that the mask is larger than the final image, so that errors in the mask (reticle) are demagnified by the demagnification ratio when they print on the final wafer image. Conventionally, a demagnification ratio of 4:1 is used in electron projection lithography.
In currently preferred technology, the EPL mask is formed in a very thin membrane that is selected with a thickness of less than one micron to reduce the heat load of energy deposited by electrons that are not part of the image. In contrast to a stencil mask that absorbs the unwanted beam, such as is used in photon (optical) lithography, the electron mask scatters the unwanted electrons only slightly, and subsequent optical elements remove the unwanted electrons from the beam.
EPL masks may be formed of any convenient material. Silicon is preferred because it is durable, its properties are well known and semiconductor techniques can be used to form the desired pattern in the material.
An alternative technology, referred to as a stencil mask, employs a mask thickness of one to three microns that absorbs the unwanted electrons.
Both of these competing technologies employ masks that are fragile and are susceptible to distortions even when they do not break. This extreme susceptibility arises in part from dimensional changes or distortions that will be reflected in changes in the final image.
It is essential, therefore that the masks be held in fixtures that do not distort them, both when being handled and also when in use.
Further, it is essential that the masks be clamped in an identical manner during mask formation and in use.
At the present state of development, nominal requirements for a mask holder are:                maintain<2 micron clamp pad coplanarity;        provide>1PSI clamping pressure;        provide<5 micron mask to mask positioning repeatability; and accommodate 200 mm diameter masks.        